The 6950 is slower than the 570. They did have one on Newegg for awhile for 199.99, but I can't seem to find it, must have gone back up in price.
PERSONALLY I would go with a new card, because going with a used card would be taking a chance, but hey, I have an 8800GTX that was used that I got that is still working today, as well as a 8600GT that still works.
But that call is up to you. Realistically, if you get a card that doesnt work, you can just send it back.
And it depends on the Phenom you have. If its like an 840, 940, or 955 or higher it should have no problem.